Conventional capacitors such as parallel-plate capacitors and electrolytic capacitors typically have low capacitance values in the range of 1 pF to about 1 mF. These capacitors also have relatively low energy density of, e.g., about 70 mWKg−1, making them generally unsuitable for use in wearable computing, e.g., via integration of these capacitors with clothing and apparel. In batteries, electrode surface and bulk material of the battery are involved in the charge storage mechanism, which generally increases the energy density of batteries relative to the energy density of conventional capacitors. The power density of a battery, however, is relatively low because of the charge storage mechanism of batteries is relatively slow compared to the charge storage mechanism of a conventional capacitor. Specifically, in contrast to batteries, charge storage in capacitors occurs at the surface, either in the electric double layer or redox states, and the bulk material of the capacitor does not contribute significantly to the charge storage which can make the charge storage faster compared to batteries. While this makes the energy density of a capacitor lower compared to that of batteries, the power density is relatively greater.
Carbon nanotube (CNT) and/or graphene based energy storage devices (generally called supercapacitors) typically have a high gravimetric capacitance of, e.g., about 120 F/g. Miniature energy storage devices made using CNT and/or graphene in forms such as nanotubes, yarns, forests, etc., can therefore offer both high energy densities and high power densities. The large ion accessible surface area of CNTs and graphene sheets formed as yarns, forests, and films can enable miniature high performance supercapacitors with power densities exceeding those of electrolytics, while achieving energy densities comparable to those of batteries. Capacitance and energy density can be enhanced by depositing highly pseudocapacitive materials such as conductive polymers. Yarns formed from carbon nanotubes were proposed for use in wearable supercapacitors.
These CNT based yarns, however, typically have low tensile strength e.g., of about 0.6-1.1 GPa, and CNT is generally not biocompatible. Therefore, CNT yarn based supercapacitors are not particularly well suited for making wearable supercapacitors and for integration with clothing and apparel. CNTs also typically do not withstand high temperatures, e.g., temperatures above 400-500° C., or above 2000° C. in vacuum. Therefore, many CNT-based capacitors are also not suitable for operation in high-temperature environments, where the temperature can exceed 1000° C., such as in various sensors used in drilling for oil.